Pilot signals (or training signals) are used in a wide variety of wireless transmission systems. Typically, a pilot signal is a known signal waveform that a receiving device expects to see at a predetermined position (usually the start) in a time slot or time frame of the downlink (i.e., the channel from the transmitter to the receiver). The receiving device uses the pilot signal to estimate the downlink channel impulse response and to synchronize accurately with the time slots of the downlink. This greatly improves receiver performance.
However, while a pilot (or training) signal improves the performance of the receiver, it adds additional signaling overhead that reduces the amount of bandwidth available to carry user traffic. This is true in single-input, single-output (SISO) wireless networks as well as in multiple-input, multiple-output (MIMO) wireless networks. In a SISO system, a pilot signal typically precedes user data symbols. The time slot or time slots used for the pilot signal symbols cannot be used to transmit user data symbols.
An increasing number of wireless networks are implemented as multiple-input, multiple-output (MIMO) systems that use multiple antennas to communicate with subscriber stations (also called mobile stations, mobile terminals, and the like). For example, MIMO antenna systems are used in code division multiple access (CDMA) networks, time division multiplexing (TDM) networks, time division multiple access (TDMA) networks, orthogonal frequency division multiplexing (OFDM) networks, orthogonal frequency division multiple access (OFDMA) networks, and others. In order to maximize throughput, MIMO networks use a variety of channel estimation techniques to measure the transmission channel between a base stations of the wireless network and a mobile device. The channel estimation technique used depends on the wireless network type (i.e., CDMA, TDM/TDMA, OFDM/OFDMA).
For example, in an OFDM/OFDMA wireless network, a MIMO base station may transmit a first pilot signal from a first antenna and a second pilot signal from a second antenna. A subscriber station receives both pilot signals and uses each pilot signal to perform channel estimation for each antenna. The symbols of the pilot signals are transmitted on orthogonal subcarriers to prevent the pilot signals from interfering with each other. No data symbols are transmitted on the subcarriers of the pilot signals. If more than two antennas are used, each antenna transmits a separate pilot signal on a set of dedicated subcarrier that is orthogonal to the other pilot signal subcarriers. The drawback to this method is that a large amount of signaling overhead is used for channel estimation. This wastes bandwidth and reduces system capacity.
A MIMO base station may also transmit separate pilot signals from different antennas using time multiplexing to separate the pilot signals. For example, during a first transmit time instant, the base station may transmit a first pilot signal from a first antenna. Then, during a second transmit time instant, the base station may transmit a second pilot signal from a second antenna. If more than two antennas are used, each antenna transmits a separate pilot signal during a separate transmit time instant. The drawback to this method is that more time slots are needed for pilot signals, thereby reducing the number of time slots available for transmitting user data. As before, this reduces system capacity. It should be noted that the above-mentioned frequency multiplexing and time multiplexing of transmitting pilot signals can be used simultaneously as long as the pilot signals transmitted from different antennas do not interfere with each other (i.e. are made orthogonal both in frequency and in time).
In some CDMA systems, multiple pilot signals are transmitted simultaneously from multiple antennas. The pilots from all antennas are transmitted with preambles that use different pseudo-random noise (PN) codes. This reduces the number of time slots required for pilot signals. However, the drawback to this method is that the receiver circuitry in the mobile device must use complicated interference cancellation techniques to recover the MIMO pilot signals. This method reduces pilot signaling overhead at the cost of a more complicated pilot recovery scheme. This method also decreases the reliability of the channel estimates.
Therefore, there is a need in the art for an improved apparatus and method for performing channel estimation in a multiple-input, multiple-output (MIMO) wireless network or in a single-input, single-output (SISO) wireless network.